The present invention is directed to an optical arrangement for use with a passive infrared motion detector for monitoring a space. The optical arangement includes a concave mirror coacting with at least one directional mirror which will direct radiation from a field of vision onto the detector to create an effective curtain for monitoring at least a boundary of a space.
An optical arrangement, which has a directional mirror and at least one means for focusing the radiation from a directional mirror onto a detector, is disclosed in German A.S. No. 21 03 909, which is based on the parent application of the U.S. Pat. No. 3,703,718. For monitoring a space or for securing specific dihedral angles, one requires optical active walls as curtains, which secure surface like and which either completely separate the space or area to be secured from other spaces or areas or completely surround the space or areas. Not only in the monitoring spaces for the unauthorizing incursion of persons, but above all in securing mobile machine parts, for example, for the purpose of work security, a gap free screening is required. Light barriers cannot fulfill such a purpose. On the basis of infrared motion detectors, a possiblity is provided by the above mentioned U.S. patent for generating a conical-shaped optical active wall with special optics. This conical wall will enclose a conical shaped space and will enable monitoring of the space. Infrared radiation from the inherent volume of the wall will have its strength determined by the optical arrangement and is imaged onto an infrared detector. An electrical signal, which for example is employed for alarm purposes, is then formed from a rate of increase of the captured infrared radiation which is characteristic of a moving person. It is therefore not possible for a person to penetrate the conical shaped wall unnoticed and therefore to enter into the conical space.
For various purposes, it is desired not only to secure spaces with different geometrical shapes but to also make it possible for variable shaping of the space to be secured for the user. Particularly for work security, a monitoring device which can be universely employed and is capable of being adapted to the respective place of employment is desired. Moreover, since the space normally has a rectangular outline, the conical shape is not particularly favorable for monitoring.